(1) Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus based on electrophotography and an image forming apparatus using this developing device, in particular relating to a developing device using a mono-component developer for developing an electrostatic latent image formed on an electrostatic latent image bearer with toner as well as to an image forming apparatus using this developing device.
(2) Description of the Prior Art
As a conventional developing device of an image forming apparatus based on electrophotography, there is a configuration which adopts a non-contact development technique using non-magnetic toner with a developing roller made of a metal sleeve as a developer bearer. In this case, the developing device includes a developer quantity regulatory member for regulating the amount of the developer in order to form a thin layer of mono-component toner on the developer bearer. Known as the developer quantity regulatory member is a blade shaped thin plate-like elastic member, which is cantilevered along one side, and whose flat part on the opposite side is abutted against the developing roller.
When the developer quantity regulatory member is made small, the distance from the supported end of the thin plate to the point of abutment on the developing roller, or the free length becomes short. As a result, the spring constant that dominates the pressure of abutment becomes greater, so that the pressure of abutment will greatly change even if the set position of the developer quantity regulatory member is slightly displaced. This requires high precision in assembly in order to ensure a stable pressure of abutment.
Further, when a conventional developer quantity regulator member is used, the maximum value of pressure of abutment (the pressure of abutment in the center of the nip portion in the pressure distribution of the abutment nip portion) tends to vary along the longitudinal direction of the developer quantity regulatory member (the direction in which the axis of the developing roller extends). Accordingly, variation of the degree of toner degradation arises across the roller length after aging (long use of the developing device). As a result, there occurs the problem that density unevenness appears after aging in the solid image across the length of the roller.
Further, in the case of the above developer quantity regulatory member, in assembly of the developing device, as the pressing action of the blade against the developing roller before and after assembly, or so called “developing roller push-in quantity” increases, the maximum pressure of abutment increases proportionally to the developing roller push-in quantity.
For this reason, it is expected that the maximum pressure of abutment also varies due to variation of the developing roller push-in quantity at the time of assembly. As a result, in order to stably set and keep the desired maximum pressure of abutment with little variation, high assembly precision is needed.
Further, when the set position between the developer quantity regulatory member and the developing roller varies due to production tolerance, radial run-out of the developing roller and other factors, along the length of the developing roller, or when the developer roller push-in quantity to the developer quantity regulatory member varies along the length direction of the roller, variation in the maximum pressure of abutment between the developer quantity regulatory member and the developer roller occurs along the length direction. From this, variation in the degree of toner degradation after aging arises along the longitudinal direction. As a result, density unevenness takes place in the solid image after aging over the length of the developing roller.
On the other hand, recently, as a measure for reducing power consumption of an electrophotographic apparatus, it is demanded to reduce power consumption in the fixing process. In the fixing process, in order to achieve lower consumption of power, it is effective to lower the quantity of heat required for fusing toner, or lower the melting point of the toner.
However, a toner having a low melting point is favorable to low-temperature fixing, whereas the strength against toner stress lowers. Accordingly, in the conventional mono-component developing system, the toner is liable to be crushed and fused due to pressure receiving from the developer quantity regulatory member. Use of toner having such a low melting point further makes conspicuous the variation in the degree of toner degradation depending on the aforementioned variation of the maximum pressure of abutment.
In order to deal with this problem, for example there is a proposal of a developing device which comprises: a flexible sheet-like developer quantity regulator member having a pressing part abutting and pressing a developer bearer; and a holding part for holding the developer quantity regulatory member so that the developer quantity regulatory member takes a curved form projected toward the developer bearer in a state where the developer quantity regulatory member is not in abutment with the developer bearer (see Patent Document 1).